The present invention relates to the use of atomic emission spectrometry for the element-specific analysis of a sample, preferably a fractionated sample eluted from a separation means such as a gas chromatograph. More particularly, the invention relates to the use of an improved reagent (or scavenger) gas which provides increased selectivity for oxygen detection.
Atomic emission spectrometers equipped with plasma-excitation means are commonly used in many applications of chemical analysis for quantitative determination of the presence of particular elements in a multi-component sample. The spectrometers are also used in the analysis of the fractionated effluent from a separatory device such a gas chromatograph (GC). In that function, the atomic emission line for the desired element is monochromatically monitored and plotted as a function of time, correlatable with the period of time over which the various components of the fractionated sample pass through the plasma-forming stage of the spectrometer. The utility of this analytical technique, particularly in such applications as the monitoring of environmental pollutants, is critically dependent on the use of a spectrometer that is highly element-selective.
As used herein, the "selectivity" of a spectrometer or detector is the ability of the detector to reject a response from compounds not containing the specific element of interest. Selectivity is normally expressed as the ratio of the mass of a compound not containing the selected element necessary to produce the same chromatographic response as a mass of a compound that does contain that element.
The selectivity in atomic emission spectrometry is as high as 10,000 for most elements, provided the specific detector used has sufficiently high resolution and is capable of filtering spectral background "noise" (interfering signals from, for example, molecular emissions). The selectivity for oxygen, however, has been considerably lower than this with the analytical equipment and methods of the prior art. For example, in atomic emission spectrometry using atmospheric pressure microwave-induced helium plasma, oxygen normally exhibits selectivity of only about 10, which is too low to allow proper distinction between oxygenated and non-oxygenated hydrocarbons.
Improvements in the results available for atomic emission spectrometry in general have been made through the use of reagent (or scavenger) gases, which are usually injected into the stream of carrier gas (generally helium), which entrains the sample just before the inlet to the spectrometer. Reagent gases are used to prevent the deposition of soot on the lamp or discharge tube, which is a particular problem when carbon or sulfur compounds are in the sample to be analyzed. The most commonly used reagent gases have been oxygen, hydrogen, and nitrogen, the choice of the particular reagent for use being dependent upon its absence in the sample compounds of interest to avoid the production of background noise. U.S. Pat. No. 3,887,280, for example, discloses the individual use of each of these gases in the elemental analysis of a carbon-containing sample.
In the particular case of the detection of oxygen-containing compounds, for example, it has been reported that oxygen-to-carbon selectivity in spectrometers operating at atmospheric pressure with microwave-induced helium plasma can be increased by using hydrogen as the reagent gas. See Slatkavitz et al, Journal of Chromatography, 355 (1986), 117-126. More particularly, it is disclosed that the addition of about 0.1-0.4 volume percent hydrogen, based on the helium flow-rate, provides improved peak shape through reduction of anomalous "negative" responses. Nevertheless, oxygen selectivity is not reported to approach the levels generally achieved for other elements, and it is also indicated that negative responses are still obtained because of background oxygen which is often present as an impurity in the helium or which enters the system through air leaks. Accordingly, there remains a need for atomic emission spectrometry having improved selectivity for oxygen.